1 - Sauhagin. Rock ledge past the entrance to "Stopping the Sausage" quest (I know that's not the name, but it amuses me to say it that way). Turn left after leaving Korthos Village, leap off the cliff. As a bonus, 3 explorer areas are in that route. Just go straight east until you hit the wall, then find the closest platform to the north you can.
2 - Ice Spider. Go to the Dwarf trader's camp, and head to the cliff. Look down, there's a path leading to the water and ice below. On the ice away from the path, a spider spawns. Another explorer is down there as well.
3 - Ice Mephit. Simply go to the frozen river where you go for Necromancer's Doom quest. He spawns down there sometimes instead of two regular Mephits.
Some rares spawn all the time for some people. I always got the Spider, no matter what, but rarely got the Mephit. The Sauhagin was 50/50, generally. Rares in dungeons are specific to those dungeons; and some of them spawn always, while others randomly appear. Explorer Areas like Korthos Island are basically outside dungeons, with access to further dungeon entrances scattered in there.
It's going to get more annoying later, when you can't even reach all the rares without having a party. In Water Works, you have only two easily reached. One needs high INT, another high WIS, and two are in hidden areas so you need a good search skill. So a Rogue and Cleric pairing is generally (assuming on build, actually) the only combo able to get all of them easily.
